Cygwin TCP slow

Brian Inglis
Tue Nov 29 05:56:00 GMT 2016

On 2016-11-28 12:54, Daniel Havey wrote:
> We have had complaints from several large hardware vendors that
> Windows networking is slow for apps like iperf that are used to
> measure throughput.  Iperf on Windows is compiled against the
> cygwin1.dll.  We have root caused the problem to a couple of lines of
> code in that set SO_RCVBUF and SO_SNDBUF to about 200KB.
> The theoretical window/RTT plot for the buffer size set by Cygwin
> (0x34000 = 200KB) gives us:
> 1ms -> 1703Mbps
> 2ms -> 851Mbps
> 3ms -> 567Mbps
> 4ms -> 425Mbps
> 5ms -> 340Mbps
> 6ms -> 283Mbps
> 7ms -> 243Mbps
> 8ms -> 212Mbps
> 9ms -> 189Mbps
> 10ms -> 170Mbps
> 20ms -> 85Mbps
> 40ms -> 42Mbps
> 60ms -> 28Mbps
> 80ms -> 21Mbps
> We have confirmed this by experiment and also confirmed that the
> limitation goes away if the buffers are not manually set.  Windows has
> autotuning and when the buffers are set manually the autotuning is
> disabled.  This is causing the throughput limitation.  So we would
> like to formally ask that you please not manually set SO_RCVBUF or

See problem reports:
Provide STC, patches, attach cygcheck -svr output?
Links to downstream bug reports, testing, results?
Note that Cygwin iperf is year old 2.0.5.

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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